With the recent increase in the utilization of various electric facilities and electronic applied equipment, electromagnetic interference (EMI) has been increasing significantly. Such EMI not only induces erroneous operations and troubles on electronic/electric equipment but also is pointed out to cause troubles on the health of operators of such equipment. It is therefore required, in such electronic/electric equipment, to restrict an intensity of an electromagnetic emission within a standard or regulated range.
As a countermeasure against the aforementioned EMI, shielding of the electromagnetic wave is required. This can be achieved utilizing a property of metal not allowing the electromagnetic wave to penetrate. For example, there are employed a method of forming a casing with a metal or a highly conductive material, a method of inserting a metal plate between circuit boards, and a method of covering a cable with a metal foil. However, in CRT or plasma display panel (PDP) and the like, transparency in the display is required because the operator has to recognize a character or the like displayed on such screen.
For this reason, it is necessary to use a material having both of a high electromagnetic wave shielding property and a good optical transmittance in order to shield electromagnetic waves generated from the front face of a display such as a PDP. As such a material, an electromagnetic wave shielding plate has been used wherein a mesh made of a metal thin film is formed on a surface of a transparent glass or plastic substrate.
By the way, the PDP emits a larger amount of electromagnetic wave in comparison with the CRT, and requires a stronger electromagnetic shielding ability. The electromagnetic shielding ability can be represented, as a simple way, by a surface resistivity. For example, in a translucent electromagnetic shield material for CRTs, there is required a surface resistivity of about 300 Ω/sq (Ω/□, ohm per square) or less, while, in a translucent electromagnetic shield material for PDPs, a surface resistivity of 2.5 Ω/sq or less is required. In particular, for a consumer-use plasma television utilizing the PDPs, an extremely high conductivity of 1.5 Ω/sq or less, more desirably of 0.1 Ω/sq or less, is desired.
With respect to a required level of translucency, the overall visible light transmittance is approximately 70% or more for the CTR, and is 80% or more for PDPs. A much higher transparency is desired.
In order to solve the above-mentioned problems, various materials and methods have been hitherto proposed wherein a metal mesh having openings is used to make electromagnetic wave shielding property (conductivity) and translucency compatible with each other, as described below.
(1) Mesh on which Silver Paste is Printed
For example, JP-A-2000-13088 (“JP-A” means unexamined published Japanese patent application) discloses a method of printing a paste made of silver powder into a net form to yield a silver mesh. However, the silver mesh yielded by this method is large in line width since the mesh is formed by printing; thus, problems such as lowering of transmittance can arise. Moreover, a surface resistance value is high and an electromagnetic wave shielding ability is small. It is therefore necessary to subject the resultant silver mesh to plating process in order to make the electromagnetic wave shielding ability high.
(2) Silver Mesh in Irregular Net Form
For example, JP-A-10-340629 discloses a silver mesh having irregular microscopic network form, and a producing method thereof. However, this producing method has a problem that only a mesh having a large surface resistance value of 10 Ω/sq (low electromagnetic wave shielding ability) is obtained. Additionally, a haze is as large as ten plus several percentage or more, so as to result in a problem that the display images become obscure.
(3) Etched Copper Mesh Formed by Photolithography
There is proposed a method of etching a copper foil by photolithography to form a copper mesh on a transparent substrate, for example, in JP-A-10-41682. This method, because of permitting a micro fabrication of a mesh, provides advantages capable of producing a mesh of a high aperture rate (high transmission), and shielding even a strong electromagnetic wave emission. However, it has a disadvantage that the mesh has to be manufactured through a number of production steps. Further, because of the use of a copper foil, the obtained mesh is not black but has a copper color. Therefore, the method had a problem to cause a decrease in the image contrast of the display equipment. Further, owing to an etching process, a crossing point of the mesh pattern becomes wider than the width of the line portion, and an improvement is being desired in connection with a moire problem.
(4) Conductive Silver Formation Utilizing Silver Salt
In 1,960s, there is proposed a method of forming a conductive thin metallic silver film pattern by a silver salt diffusion transfer process utilizing a silver deposition on a physical development nucleus in JP-B-42-23746 (“JP-B” means examined Japanese patent publication).
According to this method, a photosensitive material having an emulsion layer containing a silver salt is exposed to light and then developed, whereby a silver mesh can be formed. A silver thin film having a resistance of 10 Ω/sq to 100 kΩ/sq is obtained. However, this conductive level is insufficient for PDPs. Thus, even if the silver salt diffusion transferring method was used as it was, it was impossible to obtain a translucent, electromagnetic wave shielding material excellent in optical transmittance and conductivity, which was suitable for shielding electromagnetic waves emitted from an image-displaying face of an electronic display instrument.
As explained above, the prior electromagnetic shield materials and the producing methods therefor have been associated with drawbacks. Further, since such electromagnetic shield materials were very expensive, a reduction in the production cost has been strongly desired.
Further, for the display such as PDP, as a high image luminocity is required, an IQ optical transmittance close to 100% is strongly desired for the electromagnetic shield material. However, an increase in the aperture rate (a proportion of an area without the fine lines constituting the mesh) in the whole area for improving the optical transmittance reduces the conductivity to deteriorate the electromagnetic shielding effect. It is therefore necessary that in order to increase the conductivity, the resultant silver mesh is plated so as to have a low resistance.
In order to decrease the production costs, a method for improving the conductivity without conducting any plating process has been desired.